The objectives of the project are to study the assembly mechanisms of the bacteriophage T4 head, particularly the mechanism of DNA packaging into the head, control of initiation of prohead assembly, and the initiation of infection of the host cell by bacteriophage T4. Primary emphasis has been placed upon studying the mechanism of DNA packaging into the phage head. This work has included: 1) the development of an in vitro packaging system for DNA to produce infectious virus particles; 2) electron microscopic investigation of head filling in conjunction with thin sectioning of infected cells, and investigation of genetic requirements for filling in vivo; 3) enzymatic work on enzymes induced by T4 specific for packaging; and 4) structural work on the head and the organization of DNA within the capsid; 5) structural requirements in the duplex DNA essential for initiation or completion of packaging.